OpenArena is an open-source content package for Quake III Arena licensed under the GPL, effectively creating a free stand-alone game. You do not need Quake III Arena to play this game.
There are a lot of games out there, trust me I've spent time finding them for myself and my kids.
That should get you started altho a google search with the terms "freeware" and "mac" will find you lots and I know some games like Marathon have been released free by open source I think.
Shop for your Apple, Mac, iPhone and other computer products on Amazon.
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.